There are a number of ways to remove NOx (i.e., oxides of nitrogen) from exhaust gas of an engine. One way is to use selective catalytic reduction. In particular, a selective catalytic reduction catalyst (i.e., an SCR catalyst) positioned in an exhaust gas stream causes a reaction between NOx present in the exhaust gas and a NOx-reducing agent (e.g., ammonia) to reduce and thus remove the NOx from the exhaust gas.